Software rate adaptable modem with forward compatible and expandable functionality and method of operation

ABSTRACT

A high speed software modem is described which uses a selectable, desirable portion of the total available bandwidth of a transmission channel. In a preferred embodiment, the invention is implemented in a personal computer connected to an upstream transceiver through a channel supporting an Asymmetric Digital Subscriber Loop (ADSL) standard. The achievable target data rate of the modem is based on the capabilities of an analog front end (AFE) used in the modem, and a host processor within such personal computer. In particular, the modem AFE contains subband filtering which causes an upstream transceiver to use only a selected number of available sub-channels for downstream data transmission. The data rate of the modem is increased by upgrading the AFE or the host processor in order to increase the number of processable transmitted downstream sub-channels.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is related to the following additionalapplications, all of which are being filed concurrently herewith:

Attorney Docket No. ITE 97-001 entitled "Forward Compatible AndExpandable High Speed Communications System & Method of Operation."

Attorney Docket No. ITE 97-002 entitled "Rate Adaptable Modem WithForward Compatible and Expandable Functionality & Method of Operation."

Attorney Docket No. ITE 97-004 entitled "Modular Multiplicative DataRate Modem & Method of Operation."

Attorney Docket No. ITE 97-005 entitled "Device Driver For RateAdaptable Modem With Forward Compatible and Expandable Functionality"

Attorney Docket No. ITE 97-006 entitled "User Controllable ApplicationsProgram For Rate Adaptable Modem With Forward Compatible and ExpandableFunctionality."

FIELD OF THE INVENTION

The invention relates generally to an improved high-speed communicationssystem which establishes a data link using only a selectable portion ofthe total available bandwidth (potential downstream data transmission)of a channel. The present invention permits the characteristics(including target data rate) of the link to be controlled based on thecapabilities of an analog front end (AFE) and the processing power of ahost system incorporating the communications system. In this manner, thesystem is forward expandable in functionality, and permits a user toincrease throughput to the point of achieving full potential of theavailable chanel bandwidth.

BACKGROUND OF THE INVENTION

Remote access and retrieval of data and information are becoming moredesirable and common in both consumer and business environments. As dataand information transfer is becoming more and more voluminous andcomplex, using traditional data links such as voice-band modems is tooslow in speed. For example, the use of the Internet to locate and accessinformation is increasing daily, but the retrieval of typical graphics,video, audio, and other complex data forms is generally unsatisfyinglyslow using conventional voice-band modems. In fact, the slow rate ofexisting dial-up analog modems frustrates users, and commerce andinteraction using the Internet would have been even higher were it notfor the unacceptable delays associated with present day accesstechnology. The ability to provide such desired services as video ondemand, television (including HDTV), video catalogs, remote CD-ROMs,high-speed LAN access, electronic library viewing, etc., are similarlyimpeded by the lack of high speed connections.

Since the alternatives to copper line technology have provenunsatisfactory, solutions to the high speed access problem have beenfocused on improving the performance of voice band modems. Voice bandmodems operate at the subscriber premises end over a 3 kHz voice bandlines and transmit signals through the core switching network; the phonecompany network treats them exactly like voice signals. These modemspresently transmit up to 33.6 kbps over a 2-wire telephone line, eventhough the practical speed only twenty years ago was 1.2 kbps. Theimprovement in voice band modems over the past 20 years has resultedfrom significant advances in algorithms, digital signal processing, andsemiconductor technology. Because such modems are limited to voicebandwidth (3.0 kHz), the rate is bound by the Shannon limit, around 30kbps. A V.34 modem, for example, achieves 10 bits per Hertz ofbandwidth, a figure that approaches the theoretical Shannon limits.

There is a considerable amount of bandwidth available in copper lines,however, that has gone unused by voice band modems, and this is why aproposal known as Asymmetric Digital Subscriber line (ADSL) wassuggested in the industry as a high-speed protocol/connectionalternative. The practical limits on data rate in conventional telephoneline lengths (of 24 gauge twisted pair) vary from 1.544 Mbps for an18,000 foot connection, to 51.840 Mbps for a 1,000 foot connection.Since a large proportion of current telephone subscribers fall withinthe 18,000 foot coverage range, ADSL can make the current copper wireact like a much "bigger pipe" for sending computer bits and digitalinformation (like movies and TV channels), while still carrying thevoice traffic. For example, an ADSL modem can carry information 200times faster than the typical voice band modem used today.

ADSL is "asymmetric" in that more data goes downstream (to thesubscriber) than upstream (back from the subscriber). The reason forthis is a combination of cost, demand, and performance. For example,twisted pair wiring coupling increases with the frequency of the signal.If symmetric signals in many pairs are used within a cable, the datarate and line length become significantly limited by the coupling noise.Since the preponderance of target applications for digital subscriberservices is asymmetric, asymmetric bit rate is not perceived to be aserious limitation at this time. Therefore, the ADSL standard proposesup to 6 Mbps for downstream, and up to 640 kbps for upstream. Forexample, video on demand, home shopping, Internet access, remote LANaccess, multimedia access, and specialized PC services all feature highdata rate demands downstream, to the subscriber, but relatively low datarates demands upstream. The principal advantage is that all of the highspeed data operations take place in a frequency band above the voiceband, leaving Plain Old Telephone Service (POTS) service independent andundisturbed, even if an ADSL modem fails. ADSL further provides aneconomical solution for transmission of high bandwidth information overexisting copper line infrastructures.

Specifically, the proposed standard for ADSL divides the availabletransmission bandwidth into two parts. At the lower 4 kHz band, ordinary(POTS) is provided. The bulk of the rest bandwidth in the range from 4kHz to about 1 MHz is for data transmission in the downstream direction,which is defined to be from the exchange to the subscriber. The upstreamcontrol channel uses a 160 kHz band in between. The signals in eachchannel can be extracted with an appropriate band-pass filter.

A DMT implementation of ADSL uses the entire available 1 MHz range of acopper phone line. It merely splits the signal into 255 separatechannels, and each 4 kHz channel can be made to provide a bit rate up tothe best present day voice band (33.6 kbs) modems. This resultsessentially in overall performance which is equivalent to around twohundred V.34 modems used in parallel on the same line. Because eachchannel can be configured to a different bit rate according to thechannel characteristics, it can be seen that DMT is inherently"rate-adaptive" and extremely flexible for interfacing with differentsubscriber equipment and line conditions.

A number of problems arise, however, in attempting to implement a fullscale ADSL transceiver cost-effectively.

First, to achieve this high bit rate transmission over existingtelephone subscriber loops, advanced analog front end (AFE) devices,complicated digital signal processing techniques, and high speed complexdigital designs are required. As a result, this pushes currenttechnology limits and imposes both high cost and power consumption. Forexample, AFE devices in modem applications provide the interface betweenanalog wave forms and digital samples for digital hardware/softwareprocessing. In high speed modem technologies such as ADSL, AFE devicesneed to operate at a very high sampling rate and high accuracy. Forexample, the DMT technology has a spectrum of 1 MHz and requiressampling above 50 MHz if a sigma-delta analog-to-digital (ADC) method isused. This thus requires the state-of-art ADC technology and imposes ahigh cost for end users.

Second, the time domain signal in ADSL/DMT transmissions is a summationof a large number of carriers modulated by quadrature amplitudemodulation (QAM). This typically results in a large peak-to-peakdeviation. As a result, even though a high speed AFE is made possible, alarge dynamic range and high resolution AFE is required at the same timeto minimize quantization errors.

Third, in addition to the high sampling rate and resolution requirementfor ADSL AFEs, the other hardware and software in ADSL environment alsoneeds to operate at a much higher speed than current conventional modemcounterparts. For example, to implement the DMT technology in software,a custom and dedicated digital signal process (DSP) of a power ofseveral hundred MIPS (millions instructions per second) is required toprocess many components such as error encoding and decoding, spectrumtransforms, timing synchronization, etc. As with the AFE part of thesystem, this high speed requirement for the signal processing portion ofADSL also results in less flexible, high component costs.

Fourth, requiring a communications device (such as a modem) to fullysupport the total throughput of a standard such as ADSL may beinefficient in some cases, since many prospective users of high-speeddata links may not need to use all the available bandwidth provided bysuch standards. It is generally more preferable therefore to permitusers to throttle or scale the data throughput in a manner they cancontrol, based on their particular application needs, hardware costbudget, etc. For example, a full-scale ADSL system may have theperformance level of 200 times conventional V.34 modems, but it isapparent that even a performance improvement of 10-20 times than presentday available analog modems would be sufficient for many consumerapplications, such as Internet access and similar uses. Thus, unlikeconventional analog modems, which are available in various speedsvarying generally from 14.4 to 56 Kbps, there are no known ADSL modemswhich offer scalable performance levels to users.

Fifth, in addition to the implementation challenge, the T1E1.4 ADSLstandard does not specify the system interface and user model. Althoughvarious high level interface to support T1/E1, ATM, etc. have beendescribed, system integration with high level protocols such as TCP/IPand interface with computer operating systems have not yet been defined.As a result, there is uncertainty how existing and future modem-basedapplications can work with the ADSL technology. For example, when usersrun an Internet application which sends and receives data to and from anInternet service provider (ISP), a mutually agreed protocol is requiredto set up a call and transfer data. Possible protocols available atvarious levels include ATM (asynchronous transfer mode), TCP/IP, ISDN,and current modem AT commands. Either one of these or a possibly newprotocol needs to be defined to facilitate the adoption of ADSLtechnology.

SUMMARY OF THE INVENTION

An object of the present invention therefore is to provide acommunications system which is fully compatible with high speed, rateadaptable protocols such as are used with ADSL, but which system isnevertheless implementable with simpler analog front endreceiving/transmitting circuitry and is thus reduced in cost andcomplexity;

A further object of the present invention is to provide a communicationssystem which is fully compatible with high speed, rate adaptablemodulation protocols such as used with ADSL, but which system isnevertheless implementable with simpler digital signal processingcircuitry and is thus reduced in cost and complexity;

Another objective of the present invention is to provide a method fortransmitting data within a fractional, desirable portion of availablebandwidth in a channel by modulating only a limited number of desirablesub-channel data carriers, so that a high speed data link can beachieved that is faster, and has reduced computation and hardwaredemands;

Yet a further objective of the present invention is to provide acommunications system with smaller peak-to-peak deviation in thesub-channels signals, so as to reduce the dynamic range required for thefront end ADC, and to minimize quantization errors.

Another objective of the present invention is to provide a high speedcommunications system having a data throughput that is easilycontrollable and expandable, so that the performance range of suchsystem can be configured to any fractional percentage of total bandwidthavailable in a transmission channel, up to and including full bandwidthuse of the channel;

A related objective of the present invention is to provide a high speedcommunications system that is modular so that forward compatible andexpandable functionality can be incorporated flexibly and with a minimumof effort on the part of a user of such system;

Yet a further objective is to provide a system that is compatible withhigh speed protocols used in ADSL, but which is also easily adaptable tosupport preexisting high level data protocols, including those presentlyused for controlling high speed voice band modems;

A further object of the present invention is to provide a high speedcommunications system that self-calibrates its own performance level,based on the processing power available to such system;

Another objective of the present invention is to provide a high speedcommunications system that permits a user to configure the performanceparameters of such system using conventional personal computer hardware,software and operating systems;

A further object of the present invention is to provide an interfacebetween a host operating system and a high speed communications systemthat provide forward compatible and expandable functionality;

An additional aim of the present invention is to provide an improvedsystem for concurrent control of conventional voice data traffic on aPOTS channel, and upstream/downstream communications on separatesub-channels;

These objects and others are accomplished by providing a communicationssystem that permits a host processing device to receive selected datawithin a narrow bandwidth from an upstream transciever which can andnormally transmits a large bandwidth analog data transmission signalthrough a connected channel. A channel interface circuit AFE samples thereceived analog signal to generate a digital signal. Only a limitedportion of the bandwidth may be sampled, thus reducing front endcomplexity. A digital signal processing circuit then extracts theselected data from this limited digital signal, which is significantlyeasier to process than a full bandwidth digital signal. Feedbackinformation is provided back to the upstream transmitter which causesthe upstream transmitter to transmit downstream data thereafter onlyusing the limited bandwidth of the front end, and not the fullbandwidth. This feedback information contains information about thechannel that suggests to the upstream transmitter that the otherbandwidth in the channel is unusable. In this manner, the upstreamtransceiver is trained to accommodate the lower rate downstreamtransceiver in a manner that nevertheless preserves protocol integrity.

In a preferred embodiment, the large bandwidth analog data transmissionsignal is comprised of a number of DMT modulated sub-channels, and ananti-aliasing filter on the front end of the the downstream transceiverensures that only a limited number of such sub-channels are processed bya DMT signal processing core. The feedback information consists ofnon-zero SNR information for the selected sub-channels, and asub-channel blackout "mask" to eliminate the potential use of othersub-channels. The feedback information is sent by way of a front endtransmitting circuit which transmits an upstream data transmission usinga second frequency range different from the downstream transmission.

One implementation of the aforementioned high speed system is in apersonal computer, so that the signal processing can be accomplishedusing a processor within such computer, which in a preferred embodimentis an X86 compatible processor. Another implementation of theaforementioned high speed system uses a dedicated signal processor fordemodulating the selected sub-channels. This cuts down on processingoverhead requirements for a host processing system incorporating thesystem. In such implementations the portion of the downstream datatransmission to be processed for data extraction can be configured by auser of such systems, or alternatively, it can be dynamically determinedbased on an evaluation by the digital signal processing circuit ofperformance characteristics of different portions of the frequencyspectrum within the bandwidth potential of the upstream transceiver.

In another variation, the data rate of a system such as described abovecan be increased by processing data from an additional second limitedfrequency bandwidth portion of the total available downstream bandwidth.In a preferred embodiment, this can be done by including a number ofanti-aliasing filters in a modular bank as part of the analog front endsection, each of which passes a different frequency bandwidth portion.By making the analog front end modular, the data rate of the overallsystem can be scaled in a controllable and cost-effective fashion. Atthe same time, each analog front end portion can be operated at a slowersampling clock and smaller dynamic range. This results in a more relaxedspeed requirement and smaller quantization noise at a given number ofbits per sample.

The present disclosure also includes an interface to an operatingsystem, to facilitate controlling the high speed communications systemwhen it is incorporated in a personal computer system. This interfaceensures that the operating system treats such communications systemessentially the same as other prior art voice band modems, and in apreferred embodiment, is a device driver for the Windows NT operatingshell. Finally, the present disclosure also describes an applicationsprogram which permits a user of a personal computer to control theperformance characteristics of the high speed communications system bysetting certain system parameters when such system is incorporated in apersonal computer system. This program includes an auto calibrationroutine for setting such system parameters, or alternatively a user ofsuch program can tailor the settings subject to confirmation of theefficacy of such settings based on an evaluation of the processing poweravailable to such user.

Although the inventions are described below in a preferred embodimentimplementing the ADSL standard, it will be apparent to those skilled inthe art the present invention would be beneficially used in any highspeed rate-adaptable applications.

It should be noted that while some prior art devices also have limitedmechanisms for achieving a reduction of nominal or peak transmissionspeed in a channel, they only activate or implement such mechanisms as afallback response to a failure in the channel, or because of atransmission rate reduction in the upstream transceiver. Unlike thepresent invention, such prior art modems, during an initializationprocess, attempt to establish the highest possible transmission rateachievable by the channel and the upstream transciever. In other words,any rate reduction imposed by the downstream modem is typicallyconsidered an unintended and undesirable side effect of bad channelcharacteristics, and not a desirable and intentional design target asset forth in the present invention. In addition, the data rate reductionin such modems is accomplished primarily by varying the number of bitsper baud (hertz) at a fixed frequency, and not by controlling theoverall frequency spectrum of the downstream data transmission.Moreover, in such prior art systems, no effort is made to measure,identify or use an optimal portion of the usable bandwidth or set oftransmission sub-channels. Instead, such prior art systems typically usewhatever available bandwidth or sub-channels happen to be usable at thatinstant in time.

Similarly, while a fixed 300 baud rate downstream modem can work with anupstream 33kbs rate modem this arrangement is also unlike the presentinvention. This is because, again, the bandwidth reduction in such priorart device is so large that it is considered commercially unusable bytoday's standards. Furthermore, the smaller bandwidth modem is notcompatible with, and does not support, the higher protocols of thehigher bandwidth modem, which is also undesirable from an implementationstandpoint. Stated another way, unlike the present invention, the lowerend modem limitations of prior art system force the data link to be setup using a low level protocol that does not take advantage of the fullcapabilities of more advanced protocols.

Finally, there is no mechanism for users of either of the prior artsystems noted above to expand the functionality of such modems in acontrolled, flexible, and modular manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial depiction of the ADSL/DMT bandwidth allocation forupstream and downstream data in a channel based on frequency divisionmultiplexing (FDM) configuration.

FIG. 1B shows the relationship between a sub-band filter and an analogto digital converter that can be used in an analog front end (AFE) ofthe present invention;

FIG. 1C is a pictorial depiction of a SNR curve for a typical subscriberloop channel using sub-channel modulation;

FIGS. 1D-1G are mathematical modellings and charts that further explainthe underlying physical premises of the present invention based on DMT;

FIG. 2 is a block diagram of a general implementation of acommunications system employing the present invention, adapted for usein an ADSL environment;

FIG. 3A is a block diagram of a dedicated hardware implementation of acommunications system employing the present invention, also adapted foruse in an ADSL environment;

FIG. 3B is a block diagram of a mixed hardware and software basedimplementation of a communications system employing the presentinvention, also adapted for use in an ADSL environment;

FIG. 4 is a block diagram depicting the general structure of the datapump device driver used in the mixed implementation shown in FIG. 3;

FIG. 5 is a flowchart depicting the general operation of the control andapplication interface used in the mixed implementation shown in FIG. 3;

FIG. 6 is a block diagram of an implementation of a communicationssystem employing the present invention, also adapted for use in an ADSLenvironment, in which it is depicted how a user can modularly expandthroughput capability by adding additional AFE stages to process agreater percentage of the available bandwidth in the channel.

DETAILED DESCRIPTION OF THE INVENTION

While some of the concepts set forth immediately below are well-known, abrief explanation of ADSL technology is provided with reference to FIG.1 to facilitate an understanding of the present invention. As explainedabove, it is well-known in the art to use DMT to effectuate the ADSLstandard. In contrast to most modulation schemes, such as AM/FMtransmissions that use one carrier, DMT uses multiple carriers totransmit data bits. Specifically, T1E1.4 ADSL standards specify an up to255 channels for downstream transmission from the central office tosubscribers and up to 31 channels for upstream transmission fromsubscribers to the central office. As shown in FIG. 1, each carrier hasa bandwidth of 4.3125 kHz. The total bandwidth is 1.1 MHz for a total of255 channels. In the upstream direction, a "pilot" tone in theapproximate range of 69 kHz, is used for maintaining timingsynchronization. A similar pilot tone is transmitted in the downstreamdirection in the vicinity of 276 kHz.

Since upstream and downstream transmissions are over the same 2-pairtwisted wire, they need to be separated by either echo cancellation (EC)or frequency division multiplexing (FDM). Echo cancellation allowssimultaneous transmissions in both directions but requires a complexecho canceler implementation. On the other hand, FDM uses two differentfrequency bands for separate downstream and upstream transmissions. Asshown in FIG. 1, the upstream transmission uses subchannels from channelnumber 6 to 31, and the downstream transmission uses subchannels fromchannel number 41 to 255. While the remainder of the discussion belowfocuses on an system employing FDM, it will be appreciated by thoseskilled in the art that the present invention is adaptable and can beused beneficially with echo-cancellation approaches as well.

As with most communication environments, the transmission bit rates forboth upstream and downstream communications in ADSL are not fixed butinstead are determined by the quality of the channel. In the presentinvention, a number of well-known techniques can be used advantageouslyfor setting up the initial data link. In general, these techniques workas follows: during initialization, the channel quality is measured and acertain data rate (typically a number of bits) is assigned for each DMTsubchannel; thereafter, a "hand-shaking" process is used to dynamicallyand adaptively change the bit loadings (and energy levels). The latteris often necessary because (among other things) changes may occur in theoverall channel characteristics, changes in the target bit rate may beneeded, or new bit distributions in the sub-channels may be requiredbecause of degradations in one of the sub-channels.

The quality of the sub-channel response can be measured by the receivedsignal to noise (SNR) ratio. According to the Shannon theorem, the upperlimit of the number of bits per unit Hz that can be transmitted is log₂(1+SNR). Therefore, by measuring the received SNR at the receiver end,one can determine the number of bits allocated for each subchannelmodulation. The total data throughput rate achieved by the system,therefore, is simply the sum of all the data rates of all the usablesubchannels.

According to the T1E1.4 ADSL standards, data bits are grouped andprocessed every 250 μsec. The number of bits that can be processed overone such time frame is the summation of the bits allocated for eachsubchannel determined from the previous channel response measurement.For a given number of bits assigned to a certain subchannel, quadratureamplitude modulation (QAM) is used to convert bits to a complex value,which is then modulated by the subchannel carrier at the correspondingfrequency.

The above is a merely a brief summary of the general operation of atypical DMT/ADSL communications system. The general circuits used inprior art ADSL systems, the specifics of the bit/energy loading processfor the sub-channels, the bit fine tuning process, and the details ofthe modulation of the sub-channels, are well-known in the art, and willnot be discussed at length herein except where such structures orprocedures have been modified in accordance with the teachings herein.

The full downstream data throughput of a typical prior art ADSL standardtransceiver approaches 6 Mbps, which is more than 200 times the speed ofconventional analog modem technology. This requirement was imposed sincea large part of the initial motivation to implement ADSL was to achievehigh speed multimedia communications and video teleconferencing.Nevertheless, a large number of potential users do not want or need tohave such wide bandwidth capability. For example, many potential usersof ADSL (or similar high speed loops), including many who are intendingto use such links primarily for Internet access, only need to achievedownstream transmission speeds that are in the hundreds of kilobits persecond range. This data rate is in fact achievable using only a fractionof the available bandwidth of ADSL. By processing only a fraction of theavailable bandwidth of the ADSL standard, the present invention permitsa limited but extremely useful ADSL modem to be implemented withsignificantly less expense and complexity than previously possible. Atthe same time, because the present invention has modularcharacteristics, the proposed implementation of the present inventionaffords users an easy path to forward and upward expansion of theoverall functionality of their system.

The principle behind this aspect of the present invention is as follows:As shown in FIG. 1B, if the transmission in the channel is restricted toa smaller bandwidth by an anti-aliasing filter 80, according to theNyquist sampling theorem, the sampling rate of AFE devices (such as ADC81) that perform analog to digital conversion can be significantlyreduced. Specifically, if the total downstream bandwidth is limited tosome fractional total B Hz (in a preferred embodiment using DMT in anADSL environment, B=20 DMT channels or about 86 kHz) as shown below, wecan limit the Nyquist sampling rate to around 180 kHz. This isachievable with ADCs having greatly simplified hardware and reducedperformance requirements, in contrast to the full ADSL bandwidthapproach, which processes 200 DMT channels or 900 kHz in the case offull ADSL implementation.

The total accumulated bit rate of an ADSL communications system usingthe present invention can be calculated as follows. Suppose a totalnumber of k subchannels (out of a total of M possible) are to besupported and each channel is allocated b_(k) bits for transmission. Thetotal accumulated bit rate (R) is:

    R=(Σ.sub.i=1,k b.sub.i)*4 kHz (bits/sec)

where 4 kHz is the framing rate defined by T1E1.4 ADSL standards. Ifk=20 channels and the average number of bits per channel is 6, thentotal bit rate (R) is approximately 480 kbits/sec. It can be seen thatthis fractional use of the ADSL bandwidth nevertheless provides about 9times the performance of a conventional analog 56 kbits/sec digitalmodem. The benefits of this approach of the present invention areapparent. The overall performance and cost of a high speedcommunications system can be scaled and controlled in directrelationship to the particular needs of particular users. In general,the data rate supportable by (and the relative cost of) any particularimplementation of the present invention is generally determined by twofactors: (1) the capacity of the AFE; and (2) the capacity of thehardware performing the DMT.

The capacity of an AFE is generally measured by the maximum samplingspeed it can achieve. As explained above, the sampling speed in turndetermines the upper limit of the frequency band B (in kHz) that can beobtained. At the defined channel separation of 4.3125 kHz for ADSL, thetotal number of subchannels that can be supported is less than or equalto B/4.1325. A suitable ADC can be selected, therefore, based on theparticular data rate and cost requirements of any particular user.

The other factor that limits the number of subchannels (and achievabledata rate) is the processing power available for DMT modulation anddemodulation routines. For example, a variety of performance levels(achievable data rates) are possible with well-known dedicated signalprocessing hardware, such as digital signal processors, as discussed inmore detail below with reference to FIG. 2. Alternatively, as shown inFIG. 3, if such routines are implemented primarily by software and runby a host CPU, the required processing power (MIPS) generally increasesdirectly as function of the number of subchannels that need to beprocessed. This is because, in general, most of the processings are donein serial, or a channel by channel basis. As discussed below in moredetail, the present invention makes use of a "calibration" routine forestimating the total available processing power of a user's computingsystem in order to set an upper limit of the total subchannels that canbe supported.

Irrespective of the selection of the particular AFE or signal processingtechnique used, however, another useful (but not essential) aspect ofthe present invention is that the sub-channels with the largest signalto noise ratio (SNR) within the passband are selected for datatransmission. In other words, in the preferred embodiment of the presentinvention, those k subchannels within the passband that support thelargest number of bits are used for processing. As seen in FIG. 1C, forexample, a standard two-wire subscriber line typically has a SNR curvethat exhibits extensive attenuation with higher frequencies. It can beseen roughly in this figure that while there are more than 200sub-channels provided for downstream transmission in ADSL, it istypically the case that 50% of the maximum data rate can be accomplishedusing only a much smaller percentage (than 50%) of the sub-channels.This fact is especially useful in considering some of theshared/multi-channel bandwidth embodiments discussed further below.

The present invention, therefore, permits an implementation for a highspeed data communications system that makes use of the best portion ofthe channel, while still being upwardly compatible and forwardexpandable. By these terms, it is meant that a system constructed inaccordance with the teachings herein is completely compatible with afully implemented version ADSL DMT modem. Moreover, it will be apparentto those skilled in the art that appropriate modifications specific tothe channel and data link protocols and standards can be made so thatthe present invention can be advantageously employed in non-ADSLenvironments as well. Upward compatibility and forward expandabilityrefer to the fact that systems constructed with the present teachingscan have data rates that are easily upgraded while still preserving andmaintaining compatibility with existing standards. For example, lowerend users desiring less bandwidth can achieve a satisfactory performancewith a minimum of cost, and can then upgrade the performance levels oftheir systems at later time by suitable (and preferably modular)upgrades of the AFE and signal processing hardware/software.

A system constructed in accordance with the present teachings iscompletely compatible with the full ADSL standard because of thefollowing two aspects: According to the rate adaptation featurespecified by the T1E1.4 ADSL standards, the bit rate for eachsub-channel is determined initially (and preferably dynamically on anongoing basis) by the sub-channel SNR analysis. Specifically, an ADSLdownstream receiver can inform an upstream ADSL transmitter about thequality of the transmission; the receiver can also decide the bit ratefor each sub-channel. Therefore, a downstream, partial-channel bandwidthreceiver using the present invention can (based on the speed andpassband of such receiver) supply an upstream, full-standard ADSLtransmitter with information or control signals to effectuate atransmission only in selected sub-channels. In particular, in apreferred embodiment, the upstream ADSL transmitter is provided with SNRinformation for sub-channels outside the passband that is artificiallycontrived so as to suggest to the upstream transmitter that thesesub-channels are not usable. In this manner, the downstream transmissionis limited to a certain number of subchannels within the AFE and signalprocessing capabilities of the receiver. It can be seen, nevertheless,that this scheme is completely transparent to the transmitter, therebypermitting a system built in accordance with the present teachings to befully compatible with the ADSL standard. While not possible at this timewithin the ADSL standard, it is apparent that other high-speed dataprotocols may use a control signal, instead, to provide for expresslimiting and control of the identity of the sub-channels transmittinginformation.

As the technology improves for AFE devices and DMT implementation, thenumber of subchannels supported by a system using the present inventioncan increase. As a result, such systems can upgrade completely to a fullT1E1.4 ADSL implementation using a single higher end modular replacementAFE devices, or alternatively, a number of lower end modular AFEdevices.

General Embodiment of Present Invention

The basic structure of the present invention is depicted generally inFIG. 2. In general, the present invention can be embodied in differentcombinations of hardware and software. The primary difference betweenthese embodiments is the specific implementation of the DMT core. Thesespecific embodiments are described in more detail below with referenceto FIGS. 3A and 3B.

The structure and operation of ADSL transceivers is well-known in theart, and for that reason the present description primarily details thoseaspects of such transceivers which are necessary to an understanding ofthe inventions herein. As seen in FIG. 2, a channel 100 is made of aregular copper wire "loop", and each such loop may have differingelectrical properties, transmission lengths (sizes), varying attenuationcharacteristics, and a number of impairments or interferences. Splitter210, a conventional and well-known circuit, separates a DMT signaloccupying more than 200 sub-channels from a lower end 4 kHz POTS analogsignal. The latter can be used for simultaneous voice or conventionalanalog modem. Hybrid circuit 220 is also well-known in the art, andconsists primarily of conventional transformers and isolation circuitryused in a wide variety of high-speed devices interfacing to standardtelephone lines. A ring detect logic circuit 290 can also be implementedusing accepted techniques, to alert a Control Interface 295 to theexistence of a transmission signal originating from an upstreamtransceiver (not shown).

The full bandwidth signal is either low passed or bandpass limited to afrequency width B by suitable, well-known techniques as it passesthrough bandpass Filter and Analog/Digital Converter 280, so that only afraction of the signal in the frequency domain is passed on to Bufferand DMT Receive Core 260. Again, the only important consideration forSubband Filter 280 is that it must constrain the bandwidth of theincoming signal to be ≦B, where the sampling rate of the Analog/DigitalConverter is ≧2B. This can be accomplished by using well-known filterdesigns. By suitable selection of circuitry for Filter and ADC 280, theoverall system cost and performance can be scaled accordingly. In apreferred embodiment, the signal passed through Filter and ADC 280occupies a spectrum between approximately 200 and 400 kHz. Thisselection is based primarily on an expected average performance of atypical two-wire line. It will be apparent to those skilled in the artthat different bandpass widths and regions may be more suitable oroptimal for other kinds of data links, or other kinds of multi-carriermodulation schemes.

Moreover, in some instances, while it is somewhat more expensive toimplement, an adaptive or tunable filter may be substituted, such thatthe target frequencies of the passband are adjustable uniquely for eachnew data link. In such cases, the bandpass can be configured to coincidewith the sub-channels having the highest achievable SNR, including thesubchannels that must be supported for protocol or other system overheadreasons. Also, in some applications, the analog-to-digital conversionmay be performed by a digital signal processor, or by the host computerand therefore, the sampling rate can be dynamically controlled andmatched to the bandpass target frequency and frequency breadth. Thisfeature, in turn, would assist dynamic scaling of the data throughputbased on system computing power and overhead requirements.

Furthermore, in this preferred embodiment, using a multi-carrierapproach implementation for ADSL, a pilot tone at 276 kHz must beallowed within the passband. It is apparent that other protocols mayrequire similar pilot tones, and the design of comparable filters toachieve the functionality of Filter and ADC 280 is well within theordinary skill of one in the art.

DMT Receiver Core 260 is generally responsible for monitoring andmeasuring the SNR of the sub-channels falling within the frequency rangepassed by FILTER and ADC 280, and for extracting the original datastream from the numerous sub-carriers. In a preferred embodiment,Control Interface 295 receives system configuration information from ahost 298. This information may contain such parameters as targetthroughput rate R, target error rate, target center frequencies F forFILTER and ADC 280, target frequency width B, etc. By evaluating the SNRand bit capacities of the sub-channels computed by DMT Receiver Core260, and taking into consideration the target data rate R, ControlInterface 295 can select a number k of sub-channels up to and includingthe total available number M of sub-channels to carry the data streamfrom the upstream transmitter (not shown). The number of sub-channelsthat can be used for carrying data is directly related to the bandpassfrequency B as described above. In a preferred embodiment, M=200+(ADSL)and Control Interface 295 will usually configure k=20.

For every sub-channel other than the selected k sub-channels, a "mask"or blackout control/feedback signal is generated and transmitted by DMTTx Core 250, Buffer 240 and DAC 230 to the upstream transceiver. Thisensures that any subsequent data transmissions by the upstreamtransceiver only use the selected k sub-channels. This feedbackinformation is provided, therefore, irrespective of the transmittingcapacity of the upstream transceiver, and even during times when thechannel 100 is capable of supporting more than k sub-channels. In thismanner, the present system is perceived by upstream transceiver to becompatible with protocols and performance characteristics of theupstream transceiver, because the upstream transmitter receives feedbackinformation indicating merely that the two systems are connected througha channel with substantial signal attenuation characteristics for datasignals outside the k sub-channels. Based on the inherent rateadaptiveness of ADSL and other similar protocols, the upstreamtransceiver will automatically train itself to use only the ksub-channels predetermined by the downstream transceiver. It should benoted that the DAC 230 and Buffer 240 in the front end transmittingcircuit preferably transmit any upstream data transmissions using asecond frequency bandwidth different from that of the downstream datatransmission. However, this is not necessary in systems usingecho-cancellation. Furthermore, in ADSL applications, the size of thisbandwidth is considerably smaller, and uses only L sub-channels, whereL<M. In other xDSL applications, L may be on the same order or largerthan M.

Again, while the ADSL standard fixes the data error rate to be 10⁻⁷, itis conceivable that other applications of the present invention maytolerate a reduced error rate. For example, if maximum data throughputis required (i.e., the margin is less constrained) then the largest bitcapacity sub-channels within B can be selected. Alternatively, if thesystem is error-performance driven and has more relaxed throughputrequirements, than the 20 subchannels with the best margin are selected.A suitable combination of sub-channels can be selected by one skilled inthe art based on the particular system requirements which may vary fromapplication to application. Moreover, Controller Interface 295 mayoptimize the desired sub-channel mix dynamically depending on the typeof data transmitted in channel 100. For example, streaming audio orvideo, or pictorial graphics, may require less integrity and errorperformance than other kinds of data used by n applications programsrunning on host 298. The specifics of the structure, operations andtechniques used by Controller Interface 295 are not constrained by anyrequirements of the present invention, and can be implemented in variousways well-known to those skilled in the art.

The operation of the remainder of the circuitry shown in FIG. 2 is alsorelatively straightforward and not unlike a typical multi-carriermodulation system. Control Interface 295 ensures that DMT Transmit Core250 performs bit and energy loading only for those sub-carriersnecessary to effectuate a selected host throughput rate/error ratecombination. As with the circuitry used for Filter ADC 280, thecircuitry for performing the functions of DAC 230 can be implemented inprogrammable form to allow for greater flexibility.

Finally, while not presently supported in ADSL protocols, it isnevertheless possible that the filter in block 280 can be eliminatedentirely in some applications when the sub-channel or downstreamtransmission frequencies can be configured through appropriatehandshaking or similar procedures. In other words, if the upstreamtransmitter can be configured to transmit using only a portion of thebandwidth available in the channel, the advantages of the presentinvention can still be realized, because the ADC portion of block 280can still be relatively less complex, since it will be processing at amuch slower sampling rate than that required for a full spectrumimplementation. Moreover, such an implementation would also yield thesame commensurate savings in the DMT processing core, and reducedquantization errors.

Some special features of the present invention include the fact that:

(i) unlike hardware architectures implementing a full T1E1.4 ADSLstandard, the present invention uses a filter in the front end. Asmentioned earlier, the use of this filter is to allow low speed samplingby the ADC. If suitable handshaking between the upstream and downstreamtranscievers can be effectuated to generate a reduced downtstreamtransmission, the filter can be eliminated.

(ii) standard ring detection logic is incorporated to support existingmodem features;

(iii) DMT Rx core 260 is basically implemented the same way as specifiedby T1E1.4, but with some important differences, specifically:

[a] due to subband filtering and lower speed sampling, the frequencychannels at the output of FFT (not shown) in the DMT Rx Core have afrequency shift

[b] Since not all 256 subchannels are necessarily supported by the DMTRx Core 260, actual FFT implementation can be smaller, simpler and morecost-effective;

(iv) Control logic 295 permits the system to behave essentially like aconventional analog modem, and is used to support necessary setup taskssuch as dialing and handshaking;

(v) The use of limited bandwidth from the downstream channel reduces theneed for echo-cancellation circuitry, because there is less need foroverlap between the upstream and downstream transmissions, and thisfurther reduces system complexity and cost;

(vi) Because a smaller portion of the spectrum is processed by thepresent invention, the peak-to-peak deviation of the downstream signalis reduced, and this helps to minimize quantization errors.

Dedicated Hardware Based Embodiment

FIG. 3A illustrates an embodiment of the present invention that can begenerally described as a dedicated hardware implementation. For thepresent discussion of FIG. 3A, it can be assumed that those circuitshaving like numbered references are the same and/or perform the samefunction as their counterpart in FIG. 2. For example, unless otherwiseindicated, there is no material difference between Splitter 210 (FIG. 2)and Splitter 310 (FIG. 3A).

In this embodiment, the DMT sub-channel modulation core is implementedcompletely in dedicated processing hardware. For this appliction, DMTReceiver Core 260 typically includes a digital signal processor (DSP)(not shown) and including on-board program ROM (or other suitablememory) for storing executable microcode routines for performing bit,energy and SNR measurements of the carriers in the sub-channels. In suchan embodiment, due to the power of the DSP, there is typically no needfor processing assistance from a user's host processor 398. Thisembodiment therefore may be advantageously employed where hostprocessing power limitations are a consideration.

A user of a system shown in FIG. 3A can expand the functionality (i.e.,data throughput rate and modem features) of such system by upgrading theDMT Receiver Core 260, and where necessary, the AFE 280 as well. Thesystem of FIG. 3A can be incorporated on a typical printed circuitboard. By mounting or packaging the circuits used in such blocks in anaccessible fashion, they can be replaced or supplemented much in thesame way present users of personal computers can upgrade theirmotherboards to include additional DRAM. One practical alternative, forexample, would be to have multiple available slots to accommodate newsubband pass filters for passing a greater portion of the downstreamtransmission to be processed by the DMT core logic. Other practical andsimple variations of this approach will be apparent to those skilled inthe art.

Partial Software Based Embodiment

In the above dedicated hardware embodiment, the overall speed (datathroughput) can be maximized but with less flexibility for upgrades.This is because upgrades to such a system must take the form of hardwarereplacements, which can be more costly and difficult for the user toincorporate. On the other hand, as depicted in FIG. 3B, a number ofimportant functions of a communications system can be completelyimplemented in software, in an analogous fashion to what is commonlydescribed in the art as a "software" modem. In this case, the overallspeed of the system depends on the user's processor power available athost 398, and only the AFE portion need be implemented in hardware.

The primary differences between the embodiments of FIGS. 3A and 3B arethe following: (1) implementation of DMT modulation; (2) implementationof the control and handshaking functions; and (3) implementation of thecontrol interface. As seen in FIG. 3B, DMT Receive Core 460 and DMTTransmit Core 450 are implemented in a data pump device driver by thehost system 398. In a preferred embodiment, host system 398 includessome form of multi-purpose microprocessor (such as an x86 typeprocessor) running a suitable operating system (such as Windows byMicrosoft), and is capable of executing suitable low level drivers forthe DMT modulation (FIG. 4), as well as high level application softwarefor implementing Control Interface 500 (FIG. 5). Host processor system398 communicates over a standard bus interface 385 (i.e., a PCI bus) toFront End circuitry 396 for implementing a high speed modem. As with thecircuitry in conventional analog modems, this circuitry of the presentinvention can be effectively incorporated on a PC motherboard (i.e., BusInterface 385 and Front End Circuitry 396 can be merged so that they areessentially part of host system 398) or on a separate printed circuitboard, or as a stand-alone unit physically separated from host 398.While this approach may not provide as much throughput performance, ithas the advantage of being less expensive than the pure hardwareapproach of FIG. 2, and much easier to upgrade.

In the "software" modem implementation of FIG. 3 using a typical PCrunning Windows, the DMT Tx core 450, Rx Core 460 andControl/Handshaking logic are implemented as a Windows Data Pump DeviceDriver 400, which consist of DMT routines, associated control andhandshaking codes, and an interface to kernel 480.

A more detailed characterization of a portion of host processing system398 is depicted in FIG. 4, which illustrates a preferred embodiment of adevice driver 400 as it would be constituted for a computer operatingsystem shell 480. In the present embodiment, Microsoft Windows NT isconsidered, but it is understood that other comparable environments maybe used, including UNIX, Windows 95, etc. As is well-known, operatingsystem 480 is responsible for supervising and controlling the operationof processing system 398 and all of its associated peripheral devices.Operating system 480 also includes various interactive control andgraphical application interfaces (FIG. 5) for permitting a user ofprocessing system 398 to run various applications programs, and to setup, control, configure, monitor and utilize peripheral devices such asdisk drives, printers, monitors, modems and the like.

To assist operating system 480 to interact and control such peripheraldevices, it is also well-known to use device drivers, which areessentially low-level hardware routines executed by a host processor andoperating system. A device driver is a memory image file or executablefile that contains all the code necessary to instruct a host processorto interface and drive a particular device within a computing system.Device driver 400 acts as an interface between an operating system 480(in this case, Microsoft Windows NT) and hardware 396. In this case, forexample, device driver 400 supports hardware 396 (see FIG. 3B), which isembodied in a typical printed circuit board (or external device). Theteachings herein therefore provide for a new device driver that incombination with hardware 396 operates as a "software" modem. In thismanner, operating system 480 classifies this combination as an ADSLmodem, or in other words, another typical personal computer peripheraldevice, analogous to conventional voice-band modems.

Generally speaking, device driver 400 works as follows: a user ofprocessing system 398 desiring to establish a data link to a remote sitefor transmitting/receiving data initiates such link through anapplication program (FIG. 5). Operating system 480 (FIG. 4) interpretsand services this request by passing control of this task to devicedriver 400, which first generates appropriate instructions for a DeviceInitialization 440. In a preferred embodiment, Modem card 396 isinitialized through Bus Interface 410 using conventional voice bandmodem control commands, so that the present invention is compatible withpreexisting applications programs written for controlling modems usingoperating system 480. Similarly, therefore, control and data signals areinterpreted and transmitted by operating system 480 to a Serial PortInterface 475 so that conventional modem dialing instructions andhandshaking signals can be imparted to Modem Card 396 to establish alink through channel 100 to an upstream conventional ADSL transciever(not shown). As explained above, after suitable handshaking protocolshave been completed, the upstream fully compatible ADSL transceiver willbegin transmitting data on all available M usable sub-channels. Thisdownstream data is filtered by FILTER/ADC 380 and at this time,information for only N sub-channels (N<=M) is temporarily held in Buffer370. At or before this same time, an interrupt is generated by businterface 385 and passed through device driver bus interface 410 toalert Interrupt Service Router 415 to the existence of downstream datarequiring processing. Thereafter, DMT Receive Core 460 begins processingthe downstream data stream in response to control information from ISR415. A demultiplexer 465 extracts and correlates the data in the varioussub-channels before passing it on to Serial Port Interface 470, and backto Operating System 480. In this manner, Device Driver 400 coordinateswith Modem card 396 to effectuate a sofware modem whose performance isdirectly correlated to the computing power of a processor containedwithin the host processing device.

As mentioned earlier, Device Driver 400 also contains controlinformation for configuring the number and selection of sub-channels tobe used in the particular data link established through channel 100 withthe upstream transciever during an initialization process. As alsomentioned above, this control information may be self-determined by auser of host processing system 398, or alternatively, automaticallysensed and monitored by such processing system, based on a computingperformance rating for such system determined in a calibration routine.In either event, during the initialization process (and at all timessubsequent) the upstream transceiver is induced to use such sub-channelsonly for the ensuing data transmission. This is accomplished bytransmitting SNR information that is interpreted by the upstreamtransceiver as zero for all but K≦N of the sub-channels of the driverselection. This data is passed under control of Operating System 480through Serial Port 475, Framing control 455 and DMT Transmit Core 450before being sent out to Modem Card 396 and channel 100.

It is understood, of course, that ADSL Modem 396 can also respond to arequest from a remote modem for initiating the data link. The processfor initializing the link, nevertheless, is essentially the same as thatdescribed above. Device driver 400 can also contain control logic forsupporting typical dial-up modem operations and control codes fromconventional modem application programs typically implemented invoice-band modems, such as setting up Originate/Answer modes, monitoringcall progress, performing modem diagnostics, configuringreceive/transmit buffer sizes, supporting facsimile transmissions, aswell as performing enhanced error control, data compression and flowcontrol between Modem Card 396 and Operating System 480. Device Driver400 can also support other conventional "always-on" data linkconnections as desired, such as may be found in typical ethernet networkconnections, and other dedicated applications.

Given the teachings of the present invention, the general design of theabove Data Pump Device Driver 400 is a routine task well within theabilities of one skilled in the art. The specifics of suchimplementation are not critical or essential to the present inventions,and will vary from application to application according to systemdesigner requirements, so they are not included here. Again, while thisembodiment of the present invention is set out in the context of a PCbased host processor running Windows, it will be apparent to thoseskilled in the art that above description is merely an exemplaryimplementation. The referenced DMT routines, associated control andhandshaking codes can be employed in numerous host processing/operatingsystem environments, and in a variety of different coding organizations(high level or low level processing forms) well-known in the art.

In the preferred embodiment implemented using a standard PC runningWindows, Control/Application interface 500 includes Win32 codes whichprovide standard modem utility functions and interface with Data PumpDevice Driver 400. In FIG. 5, a flowchart of the operation of theControl/Application Interface 500 can be seen, which interface isdiscussed in more detail below.

Another particularly beneficial aspect of the embodiment of FIG. 3B isthe provision of a self-determining "performance" or calibration ratingthat can be used to determine an optimal or maximum data throughputrate. In other words, the system of FIG. 3B can automatically andadaptively configure a host system 398 to a particular throughput ratebased on an evaluation of the available computing power. In a preferredembodiment, the performance rating is determined based on a calibrationroutine executed by Data Pump Device Driver 400. This routine sets atimer, and counts how many DMT frames can be processed within the giventime; this gives a relative figure of merit for the particular hostsystem in question. For each sub-channel to be added, one DMT frameneeds to be processed within a small fraction of 250 μs. Therefore, byincrementally increasing the sub-channel count, the overall effect ontotal system processing overhead can be determined. Control/ApplicationInterface 500 provides the user with control to set a threshold ofavailable host power for implementing the high speed link. Based on thisthreshold of available power (which can be nominally set to 20%) thenumber of subchannels that can be supported can be gleaned very quickly.

In view of current technology, when DMT processing is implemented insoftware, the host processing power is more likely to be the limitingfactor than the frequency band of the subband filter 80 in FIG. 1B.Nevertheless, because host processors (and especially microprocessors)are evolving in performance at a fairly rapid rate, the presentinvention affords users an opportunity to realize a high speed data linkwith performance that is controllable, and which improves whenever thereis an upgrade in the host processing system. Since many typical presentday personal computer systems have easily accessible and replaceablehost processors, users of the present invention can easily and flexiblyexpand and enhance the throughput and functionality of an ADSL modem.

An example of the flow chart for an ADSL modem application/controlprogram 500 designed in accordance with the present teachings is shownin FIG. 5. With the teachings herein, a user of host processing system398 can, for the first time, dynamically control a forward compatibleand expandable modem, such as an ADSL modem, using modem-controlapplications software that is analogous to that only previouslyavailable for voice band modems. In a preferred embodiment, ADSL ModemCard 396 is automatically detected by Operating System 480 and set up byinitialization routine 505 by Modem Device Driver 400. A separatedetection routine 510 determines whether or not ADSL Modem Card 396 hasbeen upgraded with an additional AFE (as described generally withreference to FIG. 6 below), or alternatively whether a processor used ina host system has changed. The purpose of this step is primarily todetermine whether entries in a Device Parameters Table 560 need to beupdated because of changes in computing power, front end capabilities orother parameters that may necessitate a modification of the datathroughput characterization of the overall system when used in acommunications mode.

A calibration routine 520 is then executed to determine the nominalsetup parameters for the overall system in the manner described earlier.The results from this operation are stored in Device Paramater Table 560where they then become accessible to various application programs thatmay make use of ADSL Modem Card 396 and Device Driver 400. Theinformation stored in table 560 can include any or all of the following:(a) measurements of the computing power available to the host processor;(b) measurements of the number of frames processable by the systemwithin a particular time period; (c) estimations of the expected loadingon the processing system based on demands of other applications programsand peripheral devices; (d) minimum and maximum data throughputestimations and/or targets; (e) data identifying the type of hostprocessor; (f) data identifying the number and type of AFEs in ADSLModem card 396; (g) estimations and/or target system loading ratesavailable for a datalink (i.e., maximum available processing time to beused by the system during data transmission); (h) data transmit andreceive buffer sizes; (i) interrupt or similar priority data for themodem card; (j) estimations and/or target system sub-channelutilization; (k) estimations and/or target system sub-channel bitcapacity information; etc. It will be apparent to skilled artisans thatthe above are just examples of the types of information that may bepertinent to the performance of a high speed communications system, andthat other parameters may be considered depending on the environment,application, etc. in which the present invention is used.

After performing Auto Calibration routine 520, the results of the sameare presented to the user for acceptance and verification at step 525.At this point, the user can accept the predetermined configuration dataat step 526 (i.e., such as proposed maximum and minimum throughputrates, loading rates, etc.) and this would otherwise invoke an end ofmodem setup routine 590. Should the user not want to accept therecommended parameters, a Manual Configuration routine 530 is executed.At this juncture, various system performance data can be presented tothe user for review, along with a list of modifiable system options 532.If for example, the user elects to increase the desired throughput rate,a Verification routine 540 is then executed to determine whether suchrate is reasonably sustainable within the other parameters of thesystem. If the new proposed configuration data is otherwise acceptable,then the Device Parameter Table 560 is updated, and the setup routineagain ends. Otherwise, the user is alerted by a Notification/Suggestionroutine 550, which points out the failure of the proposed configuration,and, if possible, makes suggestions to the user for modifying the systemoptions 532 so that overall compliance can be achieved within theperformance capability of the host processing system. The program thenloops back to Acceptance routine 525, and thereafter the process isrepeated until an acceptable configuration has been achieved, and anychanges have been incorporated into Device Parameter Table 560.

While some of the operational steps above are described as implementedsolely by Operating system 480 and Device Driver 400, it is understoodthat such operations occur under direction of modem applications program500, or in some cases, based on initialization routines executed by thehost processing system. Moreover, to simplify the presentation of thepresent invention, only some of the features that may be implemented aredescribed above, and many other well-known operational steps normallyassociated with setting up or monitoring modems are omitted.

As with the design of the above Data Pump Device Driver 400, the generaldesign of the Control/Application Interface 500 required to accomplishthe above functions is a routine task well within the abilities of oneskilled in the art given the teachings herein. The specifics of suchimplementation are not critical or essential to the present inventions,and will vary from application to application according to systemdesigner requirements, so they are not included here. Again, while thisembodiment of the present invention is set out in the context of a PCbased host processor running Windows, it will be apparent to thoseskilled in the art that above description is merely an exemplaryimplementation. The referenced Control/Application Interface can beemployed in numerous host processing/operating system environments, andin a variety of different coding organizations (high level or low levelprocessing forms) well-known in the art.

Multiple AFE and Lower Sampling Speed Embodiment

FIG. 6 illustrates an example of the present invention wherein a usercan achieve significantly increased data throughput using multiple lowcost, low sampling speed AFEs, generally designated 680A, 680B, 680C,etc. As described above, these AFEs may be in separate, modular form andconfigured in a bank form so that they can be incorporated convenientlyon a printed circuit board (or similar mounting) or integrated in asingle IC chip. Each AFE can be implemented in a fixed hardwareconfiguration, or individually programmed/controlled to pass a certainportion of the downstream data transmission. Assuming suitableprocessing power is available for DMT modulation/demodulation (eitherthrough a dedicated or software implementation as described above inconnection with FIGS. 3A and 3B) a user of such system can achievesubstantially expanded functionality by upgrades having performancecharacteristics and costs of their choice.

Underlying Theory of Present Invention for ADSL/DMT Applications

A discussion of the underlying theory supporting the premise of thepresent invention now follows. In particular, this section shows themathematical foundation for the use of multiple low speed AFE's tosample a full bandwidth ADSL/DMT signal. It will be apparent to thoseskilled in the art, after reading this discussion, that the presentinventions can be advantageously used in a number of rate adaptablecommunications environments, including CAP implementations of ADSL.

DMT Transmitter

To simplify the present discussion, only a subset of the DMT transmitteris considered, as shown in FIGS. 1D and 1E. The combined model thatincludes the channel response and the DMT receiver is shown below, whereonly one branch of band-pass filtering and sampling is shown forsimplicity. To further simplify, the channel response and the SFIR arecombined together.

In this subsection, we analyze the signal over one band pass filteringprocess. The result shows that the DMT signals within the band pass canbe recovered with the same use of impulse response shortening technique.With use of multiple AFE's that cover different frequency bands, all DMTsubchannels can be recovered.

IFFT

In an ADSL environment as shown in FIG. 1D, N(N=512) frequency domainvariables are transformed into the time domain by IFFT block 60 ##EQU1##Cyclic Prefix c time domain variables at the end are added to the prefixof the sequence as shown in FIG. 1D by block 70 {z_(n) }={z_(-c),z_(-c+1), . . . , z₋₁, z₀, z₁, . . . , z_(N-1) }={y_(N-c), . . . ,y_(N-1), y₀, . . . , y_(N-1) }

AFE/DAC

Discrete time domain sequence are converted by AFE 75 to the continuoustime domain waveform as follows: ##EQU2## where p_(TX) (t) is thetransmitter pulse of the AFE/DAC used, and T_(c) is the transmitter DACdock period and equal to ##EQU3## according to the DMT ADSLspecifications. Channel

With reference now to FIG. 1E, if the channel impulse response is h_(c)(t), we have ##EQU4## where p_(RX) (t)=p_(TX) (t)xh_(c) (t). BandpassFiltering If the bandpass filter 80 has an impulse response of h_(BPF)(t), ##EQU5## where p_(BPF) (t)=p_(RX) (t)xh_(BPF) (t). AFE/ADC

Let the sampling clock be T_(s) =T_(c) ×L. This means a slower samplingby a factor of L for AFE 81. Thus, ##EQU6## For causal pulse p_(BPF)(t), we have ##EQU7## Shortening FIR (SFIR) After AFE discrete timesampling, a time domain equalizer (TEQ) called SFIR 82 is used to reducethe combined discrete time impulse response to a duration smaller thanc. If the SFIR response is h_(SFIR) [n], we have ##EQU8## PhysicalMeaning of h_(tot) [n] If we perform discrete Fourier transform at block84 for h_(tot) [n], we obtain H_(tot) [ω]=H_(SFIR) [Lω]H_(BPF) [ω] whereH_(SFIR) [L-ω] and H_(BPF) [ω] are the DFT's with period 1/(LT_(c)) and1/T_(c), respectively. Their spectra can be illustrated as shown in FIG.1F for L=5.

Dropping Cyclic Prefix

By dropping the cyclic prefix of length c/L, at block 83 we consideronly s_(k) =r_(k),k=0, . . . ,N₁ -1, where N₁ =N/L.

FFT

Performing FFT at block 84 for s_(k), k=0, . . . ,(N/L)-1, one obtains:##EQU9## where we assume h_(tot),[i] is only nonzero for i=0, . . . ,c.For a given i, let us define l'=.left brkt-top.i/L.right brkt-top.; Wecan then define i'=l'L-i, where i'=0, . . . ,L-1. Therefore, i=l'L-i'From the above definitions, we have ##EQU10##

Since ##EQU11## we have ##EQU12##

Knowing that h_(tot) [i] is zero for i<0 and i>c, we have ##EQU13##

Since ##EQU14## we have ##EQU15## where ##EQU16## Relationship betweenq_(n) and x_(n) As shown in FIG. 1G if H_(tot) [n] is a bandpass filterand nonzero only in the intervals [k(N/2L), (k+1)(N/2L)] and [(2L-k-1)(N/2L), (2L-k)(N/2L)], where 0≦k<L, the possible values of m that H_(tot)[m(N/L)+n] is nonzero for 0≦n<(N/L) are as follows.

Even k

If k is even, we can have m=k/2 and 0≦n<(N/2L) so that H_(tot)[m(N/L)+n] is nonzero over the interval [k(N/2L), (k+1)(N/2L)], andm=(L-1)-k/2 and (N/2L)≦n<(N/L) so that H_(tot) [m(N/L)+n] is nonzeroover the interval [(2L-k-1)(N/2L), (2L-k)(N/2L)].

Odd k

If k is odd, we can have m=(k-1)/2 and (N/2L)≦n<(N/L) so that H_(tot)[m(N/L)+n] is nonzero over the interval [k(N/2L), (k+1)(N/2L)],andm=L-(k-1)/2 and 0≦n<(N/2L) so that H_(tot) [m(N/L)+n] is nonzero overthe interval [(2L-k-1)(N/2L), (2L-k)(N/2L)]. The above discussion isillustrated in FIG. 1G for L=3.

In another variation that can be used in the present invention, similaradvantages to those obtained by limiting bandwidth in the receivedsignal in the downstream transceiver can be obtained by also optionallylimiting the upstream data rate of the transceiver as well. In otherwords, the ADSL standard provides for 31 channels in the upstreamdirection, but many applications do not require this amount ofbandwidth. The constraints, requirements and costs associated with theDMT modulation signal processing, and DAC 330 also can be significantlyreduced by transmitting only a sub-set of the available 31 sub-channels.The determination of the appropriate sub-channels would be accomplishedin essentially the same manner as set forth above, except that theinformation on upstream sub-channel SNR usually must be determined bythe upstream transceiver, and then fed back to the downstreamtransceiver. To save time and overhead complexity, and given the factthat there is less variation in bit capacity in sub-channels in thisfrequency band, one approach also would be to simply select a fixedsub-set of such sub-channels without regard to their actual performancecharacteristics. In a software modem environment, Control/Applicationsoftware 500 would provide a user with selectable control to effectuatea restricted upstream transmission on limited sub-channels. Again, withrespect to the ADSL standard, the only requirement in this respect isthat the upstream pilot tone must also be transmitted to establish avalid data link. An optional limited "upstream" transmission can beeffectuated in a variety of ways by the circuitry already describedabove in connection with FIGS. 2 and 3. The exact details of suchimplementation will be apparent to those of skill in the art given thepresent teachings.

Although the present invention has been described in terms of apreferred ADSL embodiment, it will be apparent to those skilled in theart that many alterations and modifications may be made to suchembodiments without departing from the teachings of the presentinvention. For example, it is apparent that the present invention wouldbe beneficial used in any xDSL or high speed multi-carrier applicationenvironment. Other types of VLSI and ULSI components beyond thoseillustrated in the foregoing detailed description can be used suitablywith the present invention. Accordingly, it is intended that the allsuch alterations and modifications be included within the scope andspirit of the invention as defined by the following claims.

What is claimed is:
 1. A high speed communications transciever forcommunicating with an upstream transciever transmitting an analog signalusing M data carrying signals within a bandwidth F through a channel tosaid system, said transceiver comprising:a channel interface circuit forcoupling to and receiving said analog signal from the channel; and afront end receiving circuit for sampling the analog signal andgenerating a digital signal based on such analog signal, the digitalsignal including data from a first frequency bandwidth portion f1 ofsaid bandwidth F containing N data carrying signals, where N<M; and afront end transmitting circuit for transmitting control information tocause said upstream transceiver to transmit downstream data only usingthe N data carrying signals, said control information indicating to theupstream transceiver that: (i) only N of the M data carrying signals aredesirable for downstream data transmission, even if said channel iscapable of supporting more than N data carrying signals; and (ii) thesystem supports any transmission protocols used by said upstreamtransceiver; and (iii) that they are connected through a channel whichshould be treated by said upstream transceiver as having substantialsignal attenuation characteristics for data signals other than the Ndata carrying signals; and a bus interface circuit for transmitting thedigital signal to a host processing device.
 2. The transceiver of claim1, further including a host processor circuit in the host processingdevice for extracting selected data from the N data carrying signals. 3.The transceiver of claim 2, wherein the selection of the N data carryingsignals can be made by the host processor circuit based in part onsampling capabilities of the front end receiving circuit.
 4. Thetransceiver of claim 3, wherein the selection of the N data carryingsignals is made by the host processor circuit based in part on theprocessing capabilities available to the host processor circuit.
 5. Thetransceiver of claim 4, wherein the selection of the N data carryingsignals can be performed by a calibration routine executed by the hostprocessor circuit to determine the processing capabilities available tothe host processor circuit.
 6. The transceiver of claim 5, wherein the Ndata carrying signals can be determined by a user of the host processingdevice based on system parameter options presented to the user by anapplications program running on the host processing device, and thisselection can be confirmed by a verification routine executed by thehost processing device.
 7. The system of claim 1, wherein the selecteddata further includes data obtained from an additional second frequencybandwidth portion f2 of said bandwidth F, so that an additional numberof data carrying signals P from the M data carrying signals (N+P<M) canbe transmitted by the bus interface circuit to be processed by the hostprocessor circuit.
 8. The system of claim 1, wherein the front endreceiving circuit includes: (i) a bandpass filter for passing the firstfrequency bandwidth portion fl of said bandwidth F; (ii) and an analogto digital converter; and wherein the bus interface circuit transmitsthe N data carrying signals to a host processor circuit that includes ahost microprocessor, a programmable memory coupled to themicroprocessor, and a data extraction routine located in the memorywhich can be executed by the microprocessor.
 9. The system of claim 8,wherein a data received rate of the system can be increased by upgradingthe host microprocessor with a second microprocessor which has increasedsignal processing capabilities, and coupling this second microprocessorto the bus interface circuit.
 10. The system of claim 8, wherein thefront end receiving circuit has a programmable frequency response. 11.The system of claim 1, further including a front end transmittingcircuit for transmitting an upstream data transmission using a secondfrequency bandwidth F2 different from F, and L upstream data carryingsignals, and where L<M.
 12. A high speed communications system forreceiving selected data contained in a number of modulated sub-channelsN from an upstream transceiver capable of transmitting a number ofmodulated sub-channels M through a channel to said system using ananalog signal, said system comprising:a channel interface circuit forcoupling to and receiving said analog signal from the channel; an analogfront end circuit for sampling the analog signal and converting it to adigital signal, the front end circuit including an anti-aliasing filterand an analog to digital converter, and the digital signal includingdata only from said number of sub-channels N, where N<=M, a businterface circuit for transmitting the N sub-channels to a hostprocessing device so that selected data can be demodulated from the Nsub-channels; a front end transmitting circuit for transmitting controlinformation to cause said upstream transceiver to transmit downstreamdata only using the N sub-channels, said control information being usedfor: (i) permanently masking out all the available sub-channels exceptthe N sub-channels for any subsequent downstream data transmission fromsaid upstream transceiver; and further indicating that: (ii) the systemsupports any data transmission protocols used by the upstreamtransceiver; and (iii) also indicating that the system and upstreamtransceiver are connected through a channel which should be treated ashaving substantial signal attenuation characteristics for sub-channelsother than the N sub-channels.
 13. The system of claim 12, wherein thenumber of sub-channels N to be demodulated is based in part on samplingcapabilities of the analog front end receiving circuit.
 14. The systemof claim 13, wherein the number of sub-channels N to be demodulated isselected by the host processing device based in part on the processingcapabilities available to the host processing device.
 15. The system ofclaim 14, wherein the selection of the number of sub-channels N can be Ncan be performed by a calibration routine executed by the hostprocessing evice to determine the processing capabilities available tothe host processing device.
 16. The system of claim 14, wherein thenumber of sub-channels N can be determined by a user of the hostprocessing device based on system parameter options presented to theuser by an application program running on the host processing device,and this selection can be confirmed by a verification routine executedby the host processing device.
 17. The system of claim 12, wherein the Nsub-channels are selected by the host processor circuit based on datacarrying capacities of the M available sub-channels.
 18. The system ofclaim 12, wherein the selected data further includes data obtained froman additional second number of sub-channels P, so that an additionalnumber of sub-channels (N+P<M) can be demodulated.
 19. The system ofclaim 18, wherein the analog front end receiving circuit has aprogrammable frequency response that determines which of the N-subchannels are to be passed from the M available sub-channels.
 20. Thesystem of claim 12, wherein the analog front end receiving circuitincludes: (i) a bandpass filter for passing the N sub-channels; and (ii)an analog to digital converter; and wherein the bus interface isconfigured to be coupled to a host processor circuit that includes ahost microprocessor, a programmable memory coupled to themicroprocessor, and a demodulation routine located in the memory whichcan be executed by the microprocessor.
 21. The system of claim 20,wherein a received data rate of the system can be controlled bycontrolling the processing power of a host microprocessor in the hostprocessing device coupled to the bus.
 22. The system of claim 12,wherein the upstream transceiver uses discrete multi-tone (DMT)modulation for generating the M modulated sub-channels, and the upstreamtransceiver perceives that the system: (i) supports asymmetric digitalsubscriber loop (ADSL) transmission protocols; (ii) is comparable withT1.413 standards using a reduced number of sub-channels and a reduceddata rate.
 23. The system of claim 12, further including a front endtransmitting circuit for transmitting an upstream data transmissionusing a second set of modulated sub-channels L separate from the Msub-channels, and where L<M.
 24. A high speed communications datareceiver for receiving data at a rate X through a channel from anupstream transceiver capable of transmitting a data stream at a rate Y,the receiver comprising:a channel interface circuit for coupling to andreceiving said data stream having said rate Y; and an analog front endcircuit for data sampling a portion of the analog signal and convertingit to a digital signal having said data rate X; and a bus interfacecircuit for transmitting the digital signal at said rate X to a hostprocessing device, and for receiving a transmission control signal fromthe host processing device to cause said upstream transmitter totransmit at a data rate substantially equal to said data rate X duringany data stream transmission; and a transmitting circuit fortransmitting control information to cause said upstream transceiver totransmit downstream data using a set of N data carrying signals, which Ndata carrying signals constitute a subset of M signals available forcarrying data in the channel, said control information includingfeedback information from the host processing device indicating to theupstream transceiver that: (i) only N of the M data carrying signals aredesirable for downstream data transmission, even if said channel iscapable of supporting more than N data carrying signals; and (ii) thereceiver supports any upstream transceiver data transmission protocols;and (iii) that they are connected through a channel which should betreated by said upstream transceiver as having substantial signalattenuation characteristics for data signals other than the N datacarrying signals; and wherein data sampling circuitry requirements ofthe analog front end are reduced because data sampling is only performedfor a fractional portion k of the data stream, where k=M/N.
 25. Thesystem of claim 24, wherein the analog front end circuit furtherincludes an anti-aliasing filter for filtering the analog data signal togenerate the fractional portion of the data stream that requires datasampling and extracting.
 26. The system of claim 24, wherein an upstreamdata transmission uses a second set of data carrying signals L separatefrom the M data carrying signals, and where L<M.
 27. The system of claim24, wherein the ratio of X to Y is approximately 0.5 or less, and thisratio can be increased through modular additions to the analog front endcircuit.
 28. A method of operating a high speed communications systemthat is coupled to a host processing device and an upstream transceiverthrough a channel capable of supporting an analog data transmissionsignal having a bandwidth F, said method comprising:(a) configuring saidsystem to achieve a receive data rate X; and (b) receiving an analoginitialization signal having a bandwidth F from the upstream transceiverthrough the channel; and (c) generating a digital signal based onsampling a portion of the analog initialization signal corresponding toa first frequency bandwidth portion f1 sufficient to meet receive datarate X; and (d) transmitting the digital signal to a signal processingcircuit, so that characteristics of data carrying signals containedwithin first frequency bandwidth portion f1 can be determined; and (e)generating feedback information indicating to the upstream transceiverthat bandwidth other than the first frequency bandwidth f1 should not beused for data transmission, said feedback information further indicatingthat: (i) only N out of M available data carrying signals are desirablefor downstream data transmission, where N<M/k, and k is an integer, evenif said channel is capable of supporting more than N data carryingsignals; and (ii) the system supports any transmission protocols used bythe upstream transceiver; and (iii) that they are connected through achannel which should be treated by said upstream transceiver as havingsubstantial signal attenuation characteristics for data signals otherthan the N data carrying signals; and (f) receiving an analog datatransmission signal having a bandwidth f1 from said upstreamtransceiver; and (g) generating a digital signal based on sampling theanalog data transmission signal; and (h) transmitting the digital signalto the host processing device so that it can be processed to extractselected data from the data carrying signals.
 29. The method of claim28, further including a step: determining an optimal size and locationof first frequency bandpass portion f1 so as to minimize the amount ofprocessing required to extract the data from the digital signal.
 30. Themethod of claim 28, wherein the system transmits feedback informationcontaining intentionally altered channel characteristic information. 31.The method of claim 28, wherein the M data signals include M modulatedsub-channels, and the selected data is contained in N of the Msub-channels.
 32. The method of claim 31, further including a step:determining an optimal set of N sub-channels so as to minimize theamount of processing required to extract the data from the digitalsignal.
 33. The method of claim 32, further including a step whereindata protocol information pertaining Asymmetric Digital Subscriber Loopsis transmitted by the upstream data transceiver so as to set up an ADSLcompatible data link, which ADSL compatible data link uses a reducednumber of sub-channels and a reduced data rate from that available usingfull rate T1.413 standards.
 34. A high speed communications system forprocessing an analog data signal from a channel capable of supporting Mmodulated sub-channels, said system comprising:a channel interfacecircuit for coupling to and receiving said analog data signal from thechannel; an analog front end circuit for processing the analog datasignal and converting it to a digital signal; a processing circuit forextracting data from the digital signal, the digital signal includingdata taken from a number N of said sub-channels, where N isintentionally selected to have a value less than M, and where N isnegotiated with a remote transceiver during an initialization procedure;and further wherein during said initialization procedure, feedbackinformation from the processing circuit indicates to said remotetransceiver that: (i) only N of the M data sub-channels are to be usedfor data transmission, even if said channel is capable of supportingmore than N sub-channels; and (ii) the system supports any dataprotocols used by the remote transceiver, including xDSL basedprotocols, so that a reduced rate but otherwise compatible version ofsuch xDSL based protocols can be used in the channel; and (iii) thatthey are connected through a channel which should be treated by saidremote transceiver as having substantial signal attenuationcharacteristics for sub-channels other than the N sub-channels.
 35. Thesystem of claim 34, where the N sub-channels are initially loaded withbit capacities that are left essentially unchanged unless transmissioncharacteristics of said channel vary.
 36. The system of claim 34,wherein the selection of the N sub-channels can be done during theinitialization procedure.
 37. The system of claim 34, wherein the valueof N is based on signal processing capability of the processing circuit.38. The system of claim 34, wherein a target data rate of the system canbe increased by processing an additional number of sub-channels P fromthe M sub-channels, and where N+P<M.
 39. The system of claim 34 furtherincluding a front end transmitting circuit for transmitting saidfeedback information.
 40. The system of claim 34, wherein the upstreamtransceiver uses discrete multi-tone (DMT) modulation for generating theM modulated sub-channels, and the channel supports asymmetric digitalsubscriber loop (ADSL) transmission protocols.
 41. A high speedcommunications system for processing an analog data signal from achannel capable of supporting M modulated sub-channels, said systemcomprising:a channel interface circuit for coupling to and receivingsaid analog data signal from the channel; an analog front end circuitfor processing the analog data signal and converting it to a digitalsignal; a bus interface circuit for transmitting the digital signal to ahost processing device, and for receiving a transmission control signalfrom the host processing device to cause a remote transmitter totransmit using only a number N of said sub-channels, where N isintentionally selected to have a value less than M, and where N isnegotiated with an upstream transceiver during an initializationprocedure; and wherein said transmission control signal furtherspecifies that: (i) only N of the M sub-channels are desirable fordownstream data transmission, even if said channel is capable ofsupporting more than N sub-channels; and (ii) the system supports anydata protocols used by the remote transmitter for transmitting data insaid channel, including xDSL based protocols, so that a reduced rate butotherwise compatible version of such xDSL based protocols can be used inthe channel; and (iii) that they are connected through a channel whichshould be treated by said remote transmitter as having substantialsignal attenuation characteristics for sub-channels other than the Nsub-channels.
 42. The system of claim 41, wherein the value of N isbased on signal processing capability of the host processing device. 43.The system of claim 41, wherein a data rate of the system can beincreased by processing an additional number of sub-channels P from theM sub-channels, and where N+P<M.
 44. The system of claim 41, wherein theupstream transceiver uses discrete multi-tone (DMT) modulation forgenerating the M modulated sub-channels, and the channel supports aT1.413 compatible data link using asymmetric digital subscriber loop(ADSL) transmission protocols.
 45. A method of processing an xDSL signalfrom a digital subscriber loop, said method including the stepsof:negotiating a reduced data rate R' for said signal between adownstream and an upstream transceiver; and processing feedbackinformation from the downstream transceiver indicating that: (i) only Nout of M sub-channels available to the upstream transceiver aredesirable for downstream data transmission, where N<=M/k, and k is aninteger, even if said channel is capable of supporting more than Nsub-channels; and (ii) the downstream transceiver supports any dataprotocols, including xDSL based protocols used for data transmission bythe upstream transceiver, so that the upstream transceiver perceivesthat said downstream transceiver is fully compliant with T1.413standards, but with a reduced number of sub-channels from that availableusing a full rate ADSL transceiver; and (iii) they are connected througha channel which should be treated by said upstream transceiver as havingsubstantial signal attenuation characteristics for sub-channels otherthan said N sub-channels; and thereafter transmitting said xDSL signalfrom the upstream transciever to the downstream transceiver utilizingsaid sub-channels N to effectuate the reduced data rate R', where N isintentionally selected to be less than a maximum number of saidsub-channels M supported by said digital subscriber loop; wherein thenumber of sub-channels N is based on signal processing capabilityavailable to the downstream transceiver.
 46. The method of claim 45,wherein the data rate of the system can be increased by processing anadditional number of sub-channels P from the M sub-channels, and whereN+P<M.
 47. The method of claim 45, wherein the upstream transceiver usesdiscrete multi-tone (DMT) modulation for generating the M sub-channels.48. The method of claim 45, wherein the reduced data rate R' can bespecified by a user operating the downstream transceiver.
 49. A methodof processing an xDSL signal from a digital subscriber loop, said methodincluding the steps of:negotiating a reduced data rate R' for saidsignal between a downstream and an upstream transceiver; and processingfeedback information from the downstream transceiver indicating that:(i) only N out of M sub-channels available to the upstream transceiverare desirable for downstream data transmission, where N<=M/k, and k isan integer, even if said channel is capable of supporting more than Nsub-channels; and (ii) the downstream transceiver supports any dataprotocols including xDSL based protocols used for data transmission bythe upstream transceiver so that the upstream transceiver perceives thatsaid downstream transceiver is fully compliant with T1.413 standards,but with a reduced number of sub-channels from that available using afull rate ADSL transceiver; and (iii) they are connected through achannel which should be treated by said upstream transceiver as havingsubstantial signal attenuation characteristics for sub-channels otherthan said N sub-channels; and thereafter transmitting said xDSL signalfrom the upstream transciever to the downstream transceiver utilizing afrequency bandwidth f1 to carry said N sub-channels and to effectuatethe reduced data rate R', where f1 is intentionally selected to be lessthan a maximum bandwidth F supported by said digital subscriber loop;and wherein the frequency bandwidth f1 is based on signal processingcapability available to the downstream transceiver.
 50. The system ofclaim 49, wherein the value of f1 is based on signal processingcapability of a host processing device.
 51. The system of claim 49,wherein a target data rate of the system can be increased by processingan additional frequency bandwidth f2, and where f1+f2<F.
 52. The systemof claim 49, wherein the channel supports asymmetric digital subscriberloop (ADSL) transmission protocols.